Lignin.
Lignin is a natural amorphous polymer that acts as the essential glue that gives plants their structural integrity. It is a main constituent of lignocellulosic biomass (15-30% by weight, 40% by energy), together with cellulose and hemicelluloses. However, lignin has received little attention relative to cellulose in terms of R&D efforts to biofuel production. In 2004, the pulp and paper industry alone produced 50 million tons of extracted lignin, yet the existing markets for lignin products remain limited and focus on low value products such as dispersing or binding agents. As a result, only approximately 2% of the lignin available from the pulp and paper industry is used commercially with the remainder burned as a low value fuel. Even more lignin waste will be anticipated when large-scale cellulosic production is commercialized.
Unrealized Potential.
The potential and value of lignin as a biomass source for the sustainable production of the nation's high quality liquid fuel requirements, for example, and/or bulk chemicals is undervalued because of current technology and process limitation. The utilization of waste lignin as a feedstock for conversions to hydrocarbons offers a significant opportunity for enhancing the overall operational efficiency, carbon conversion rate, economic viability, and sustainability of biofuels production. Due to its availability, low O/C ratio, and markedly low total oxygen content compared to biomass-derived carbohydrates (˜36% versus ˜50%, respectively), lignin is a promising feedstock for production of renewable hydrocarbon fuels and chemicals. Lignin's native molecular structure is, however, approximately C800-900 which is far higher than the carbon chain lengths required for fuel applications (˜C6-20).
Therefore, lignin must be depolymerized, its H/C ratio needs to be increased, and its O/C ratio must be further decreased.
Limitations of the Lignin Process Art.
Previous attempts to decrease the degree of polymerization and/or the oxygen content of biomass have involved various transformations, including hydrothermal decarboxylation, acid-catalyzed dehydration, metal-catalyzed decarboxylation, pyrolysis, carbonization, and metal-catalyzed hydrogenation. However, there are few commercially viable conversion processes available for converting lignin to fuel-derived products. Pyrolysis, for example, can convert lignin in whole biomass to hydrocarbons, but results with poor selectivity (and as a component of whole biomass).
Traditionally, milled wood lignin (MWL) has been used as a representative source for isolating native lignin. The isolation procedure for MWL involves the production of wood meal by Wiley milling of the wood, followed by vibratory or rotary ball milling, and subsequent extraction with doxane-water. The yield of MWL varies depending on the extent of milling, ranging from 25% to 50%. However, severe chemical modification of the lignin occurs. In fact, increases in carbonyl and phenolic hydroxyl content, as well as decreases in molecular weight and cleavage of aryl ether linkages, have been reported as a result of the MWL isolation procedure. Moreover, MWL is not representative of the whole lignin in wood, but primarily originates in the secondary wall of the cell, with differences in extraction rate after ball milling due to inherent differences in the chemistry of lignin in the middle lamella and the secondary wall being responsible.
To date, virtually no approach has proven successful for converting the lignin that results from biochemical bioprocessing into hydrocarbon liquids or chemicals. Particularly, in this regard, the deconstruction of the complex lignin polymeric framework into low molecular weight reactive moieties amenable for subsequent removal of oxygen to produce hydrocarbons fuels and chemicals has proven to be challenging.